Environmental awareness is growing in the U.S. and around the world leading to increasing public and regulatory pressures to reduce pollutant emissions from boilers, incinerators, and furnaces. One pollutant of particular concern is NOx (by which is meant individual oxides of nitrogen such as but not limited to NO, NO2, N2O, N2O4, and mixtures thereof), which has been implicated in acid rain, ground level ozone, and fine particulate formation.
A number of technologies are available to reduce NOx emissions. These technologies can be divided into two major classes, primary and secondary. Primary technologies minimize or prevent NOx formation in the combustion zone by controlling the combustion process. Secondary technologies use chemicals to reduce NOx formed in the combustion zone to molecular nitrogen. The current invention is a primary control technology.
In primary control technologies, commonly called staged combustion, mixing between the combustion air and fuel is carefully controlled to minimize NOx formation. The formation of NOx from fuel nitrogen is based on a competition between the formation of NOx and the formation of N2 from the nitrogenous species in the fuel volatiles and char nitrogen. Oxygen rich conditions drive the competition towards NOx formation. Fuel rich conditions drive the reactions to form N2. Primary control strategies take advantage of this phenomenon by carefully controlling the mixing of air and fuel to form a fuel rich region to prevent NOx formation. To reduce NOx emissions, the fuel rich region must be hot enough to drive the NOx reduction kinetics. However, sufficient heat has to be transferred from the fuel rich first stage to the furnace heat load in order to prevent thermal NOx formation in the second stage.
By far the most common type of primary control device is the low NOx burner (LNB). In this device the air is typically aerodynamically staged to form a fuel rich zone followed by a burnout zone. A conventional low NOx burner includes a first zone, near the feed orifice, which is controlled by primary air and fuel, and which is very fuel rich. In a second zone, the remainder of the secondary air and any tertiary air then allow the fuel nitrogen to continue to be chemically processed to form N2 provided that the local stoichiometrics are rigidly controlled. In this region the hydrocarbons and the char are burned out. Although the LNB is a fairly inexpensive way to reduce NOx, currently available versions are not yet capable to reach the emissions limits in pending regulations. Other issues are increased carbon in the ash and reduced flame stability.
Low NOx burners represent a fairly mature technology and as such are discussed widely throughout the patent and archival literature. Many ideas have been proposed to enhance the effectiveness of LNB's while minimizing detrimental impacts such as poor flame stability and increased carbon in the ash. Of these ideas two are particularly relevant: preheating the air to the first stage, and converting the combustor to oxy-fuel firing.
Both air preheat and oxy-fuel combustion enhance the effectiveness of staged combustion by increasing the temperature in the primary zone without increasing the stoichiometric ratio. Oxy-fuel combustion offers the additional advantage of longer residence times in the fuel rich region, due to lower gas flows, which has been shown to reduce NOx emissions. As discussed above, staged combustion uses a fuel rich stage to promote the formation of N2 rather than NOx. Since the reactions to form N2 are kinetically controlled, both the temperature and the hydrocarbon radical concentration are critical to reducing NOx formation. For example, if the temperature is high and the radical concentration is low, such as under unstaged or mildly staged conditions, NOx formation is increased. When the radical concentration is high but the temperature is low, such as under deeply staged conditions, the conversion of intermediate species such as HCN to N2 is retarded. When air is added to complete burnout, the intermediates oxidize to form NOx, therefore the net NOx formation is increased. Sarofim at al. “Strategies for Controlling Nitrogen Oxide Emissions During Combustion of Nitrogen bearing fuels”, 69th Annual Meeting of the AIChE, Chicago, Ill., November 1976, and others have suggested that the first stage kinetics can be enhanced by preheating the combustion air to fairly high temperatures. Alternately Kobayashi et al. (“NOx Emission Characteristics of Industrial Burners and Control Methods Under Oxygen Enriched Combustion Conditions”, International Flame Research Foundation 9th Members' Conference, Noordwijkerhout, May 1989), suggested that using oxygen in place of air for combustion would also increase the kinetics. In both cases the net result is that the gas temperature in the first stage is increased while the radical concentration stays the same, resulting in reduced NOx formation. Further, using both air preheat and oxy-fuel firing allows the first stage to be more deeply staged without degrading the flame stability. This allows even further reductions in NOx formation.
Oxy-fuel firing offers a further advantage for LNB's. Timothy et al (“Characteristics of Single Particle Coal Combustion”, 19th Symposium (international) on Combustion, The Combustion Institute, 1983) showed that devolatilization times are significantly reduced, and the volatile yield is increased, when coal is burned in oxygen enriched conditions. These tests were single particle combustion tests performed under highly fuel lean conditions, which does not provide information on how much oxygen is needed to accomplish this under more realistic combustion conditions. The higher volatile yield means that the combustibles in the gas phase increase as compared to the baseline—leading to a more fuel rich gas phase which inhibits NOx formation from the volatile nitrogen species. In addition, the fuel volatiles ignite rapidly and anchor the flame to the burner, which has been shown to lower NOx formation. The enhanced volatile yield also leads to shorter burnout times since less char is remaining.
Although the prior art describes several elegant enhancements for staged combustion and LNB's, several practical problems have limited their application. First, preheating the combustion air to the levels required to enhance the kinetics requires several modifications to both the system and the air piping. The air heater and economizer sections must be modified to allow the incoming air to be heated to higher temperatures, which may require modifications to the rest of the steam cycle components. The ductwork and windbox, as well as the burner itself, must also be modified to handle the hot air. All of the modifications can be costly and can have a negative impact on the operation of the boiler.
The primary barrier to the use of oxy-fuel firing in boilers has been the cost of oxygen. In order for the use of oxygen to be economic the fuel savings achieved by increasing the process efficiency must be greater than the cost of the supplied oxygen. For high temperature operations, such as furnaces without significant heat recovery, this is easily achieved. However, for more efficient operations, such as boilers, the fuel savings attainable by using oxy-fuel firing is typically much lower than the cost of oxygen. For example, if a typical coal-fired utility boiler were converted from air firing to oxygen firing, approximately 15 to 20% of the power output from that boiler would be required to produce the necessary oxygen. Clearly, this is uneconomic for most boilers.
Thus there remains a need for a method for achieving reduced NOx emissions in combustion of fuel (particularly coal) containing one or more nitrogenous compounds and especially for a method which can be carried out in existing furnaces without requiring extensive structural modifications.